×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Meggering 15kV cables

Meggering 15kV cables

Meggering 15kV cables

(OP)
A contractor recently told me he was going to hi-pot some cables some time before he energizes them.  I suggested to him that he megger them at the same time, get a base line reading, then megger them again prior to re-energization and compare the readings to verify there was no damage or water infiltration into the cables.

Was this bad advice?  Thanks for any help you can offer.

RE: Meggering 15kV cables

I have always understood Hi-potting to be the high voltage equivalent of meggering.
The highest voltage megger I have seen was 2500 volts. Standard seems to be 500/1000 volts. Hi-pot sets start at about 1000 volts and go up from there.
respectfully

RE: Meggering 15kV cables

I would check with cable supplier before hi-potting any cable in the field.  I believe IEEE now does NOT recommend high-pot testing of MV cables in the field, due to risk of insulation damage.  

If I was going to hi-pot, I would megger BEFORE the hi-pot.  I  agree that a second megger just prior to energization would be a good idea.  Who knows what might be hanging out there.  

RE: Meggering 15kV cables

AC hipot is OK, DC may damage the cable. If you can provide cable details I can give you some recomendations.

RE: Meggering 15kV cables

(OP)
Okay, I found out a little bit more about this:

In the Okonite engineering handbook (p.52), it says that XLPE insulation is damaged by hi-potting and therefore dc testing was not recommended any more (after 1996), but that EPR insulation is not damaged by this testing (I only spec EPR unless otherwise required by the client).

The cables the contractor is supplying are Okonite.  Okonite gives guidelines for field hi-pot testing of their cables, but suggests that hi-potting after splicing/terminations could damage those components.

BTW, meggering has been used to identify imminent failure of high-voltage cables by establishing a baseline and then meggering on a schedule, and meggering is non-destructive.  I read an IEEE Indusry Applications article about this several years ago.

Thanks for your responses.

RE: Meggering 15kV cables

Hello tommom
It looks like I have an opportunity to learn something today.
At what voltage would you megger a 15kv cable?
Thanks
respectfully

RE: Meggering 15kV cables

(OP)
According to Okonite's guide, it's 300 volts per mil, so at 15kV class for acceptance testing:

100% (175 mil) = 55kV
133% (220 mil) = 65kV

I haven't seen 173% insulation in any catalogs for some time, and never had a reason to use it.

For maintenance testing for cables less than 5 years old:

40 kV

Testing frequency is also given in the Okonite engineering handbook.  I got mine free from the local rep.

RE: Meggering 15kV cables

Thanks tommom
Same words, different meanings.
I would call that a DC hi-pot at 40kv. At that level I agree that a baseline reference is a good suggestion.
I guess I did learn something today.
Thanks.
respectfully

RE: Meggering 15kV cables

Refer to NETA- ATS (Acceptance Testing Standards). You can buy them from www.netaworld.org.

RE: Meggering 15kV cables


Depending on the protocol the DC HIPOT and the insulation resistance test can be different or the same. According to IEEE 400 both tests are useless at finding defects on new cable systems. However, if you do not have a diagnostic test, I recommend an insulation resistance test to prove that the system is not shorted. If someone left the grounds on the cable it would be a terrible shame is someone got hurt and it could have been prevented with a simple resistance test!

Cable manufacturers give recommendations for a DC test knowing full well that it is completely useless to find 99% of defects.  They would never use a DC test to pass cable through their quality control department!  So why do they still recommend it in the field?  Soon they will not be able to make this recommendation because it is being dropped from the standards.



Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

RE: Meggering 15kV cables

(OP)
Ben Lanz,

So what IS recommended (besides an insulation resistance test) for an acceptance test?  Is there such a thing?

I was thinking of the megger more as a baseline for future maintenance.  I have seen this discussed in IEEE IAS articles (I don't have citations.)

Thanks.

RE: Meggering 15kV cables

IEEE 400-2001 still allows for a simple AC high potential withstand.  However, with this test you will miss many types of defects, you can not assure the reliability of your system, and you may unknowingly do more harm than good.

If you want to assure reliability and find defects before the contractor warranty has ended IEEE 400-2001 states:

"If the cable system can be tested in the field to show that its partial discharge level is comparable with that obtained in the factory tests on the cable and accessories, it is the most convincing evidence that the cable system is in excellent condition."

So what are the manufacturers' standards for cable systems?  These standards have over the 40 years of expericence behind them!

IEEE 48 Terminations                 No PD > or = 5pC up to 1.5Uo
IEEE 404 Joints                      No PD > or = 3pC up to 1.5Uo
IEEE 386 Separable Connectors        No PD > or = 3pC up to 1.3Uo
ICEA S-94-649 MV Extruded Cable      No PD > or =  5pC at < 4Uo

Let's not reinvent the wheel or make things complicated. Let's use the proven industry standards.

Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources